Contemporary motor starters often consist of a combination of motor circuit breaker and contactor, such as motor starters of the Eaton Corp., which combine a PKZ-type or PKE-type motor circuit breaker with a DILM-type contactor.
The use of electromechanical contactors for turning motors and electrical loads on and off, as in the motor starters explained in the preceding, has been an established technique for decades. Electromechanical contactors, whose mechanical contact elements withstand high short-circuit currents and, in the deactivated state, disconnect the electrical load from the voltage source at all poles and galvanically, may be produced economically.
Semiconductor switches—so-called solid state relays (SSR's) or solid state contactors (SSC's)—are also increasingly used for switching electrical loads. For example, these were and are used as unipolar semiconductor switches for frequent connection and disconnection of resistive loads, such as damper registers or IR power radiators. By continuously further developing power semiconductors such as thyristors and TRIAC's, these have become increasingly more voltage-stable and also increasingly have smaller power losses.
Meanwhile, pure semiconductor switches are also available as a replacement for conventional contactors for switching motors in certain power ranges, e.g., DRC-type semiconductor contactors from the company Crydom, Inc., San Diego, U.S.A., or RGCM-type semiconductor contactors from Carlo Gavazzi GmbH. Such semiconductor switches have some advantages and disadvantages as compared to conventional contactors: Advantages are, above all, the lack of mechanical wear on the switching contacts, and thus a high number of possible connection and disconnection processes, rapid connection and disconnection, and possible integrated phase switching—for example, in order to operate a motor, even in a reversing contactor circuit, in the same form factor of, in particular, a 45 mm housing width. It is disadvantageous that, when switching off, no galvanic isolation is established; in the connected state, a certain power loss, and therefore waste heat, are produced that must be removed via suitable measures, e.g., the use of heat sinks; and a lower short-circuit resistance and switching performance.